Purpose: In this study, the authors aimed to determine how between-language interaction contributes to phonological acquisition in bilingual Spanish-English speaking children. Method: A total of 24 typically developing children, ages 3;0 (years;months) to 4;0, were included in this study: 8 bilingual Spanish-English speaking children, 8 monolingual Spanish speakers, and 8 monolingual English speakers. Single word and connected speech samples were obtained for each child. This study examined interaction between the two languages of bilingual children during phonological acquisition through the measurement of (a) "transfer" (the frequency and types of phonological transfer present in the speech of bilingual children); (b) "deceleration" (a slower rate of acquisition for bilinguals as compared with monolinguals); and (c) "acceleration" (a faster rate of acquisition for bilinguals as compared with monolinguals. Results: Findings demonstrated that (a) transfer was evident in the productions of bilingual children, (b) differences were found in accuracy between monolingual and bilingual children, and (c) sound frequency did not predict differential accuracy of either phonetically similar sounds between languages or phonetically dissimilar sounds specific to Spanish or English. Implications: The results from this study indicate that transfer, deceleration, and a possible variation of the acceleration hypothesis occur in bilingual phonological acquisition. Evidence was found for separation and interaction between the bilingual children's 2 languages (J. Paradis & F. Genesee, 1996).